Digital Subscriber Line (DSL) technology provides high-speed transport of digital information over twisted pair phone lines. The typical DSL system uses a multicarrier modulation format that employs Fourier Transform as the modulation/demodulation engine. This transmission scheme is denoted as Discrete Multitone Modulation (DMT). The DSL system divides the transmission spectrum into multiple frequency bands called subchannels. Each subchannel can be modulated with a sinusoidal carrier to transmit information. A transmitter modulates an output data stream containing information bits onto one or more subchannels. A receiver demodulates all the subchannels in order to recover the transmitted information bits as an input data stream. For simplicity of reference, the term transceiver refers collectively to transmitters and receivers.
DSL systems can be used in an asymmetrical configuration (ADSL) where the data rate in a given direction, upstream from the residence to the central office and downstream from the central office to the residence, is different. DSL Systems can also be used in a symmetrical configuration where the data rate is equal in both directions.
The quality of transmission, in terms of Bit Error Rate (BER), can also be varied for different subchannels within the composite data stream. In addition to the high-speed digital transport over the twisted pair telephony wire, analog telephony, plain old telephone service (POTS), is also supported. The POTS signal occupies the low frequency region from 0-4 kHz and the digital data occupies the higher frequency band from approximately 30 kHz to many hundreds of kilohertz or several megahertz depending on the application.
ADSL enables a variety of applications, such as Internet access, digitized voice calls, video-on-demand, broadcast video, and video conferencing. Typically transceivers are designed and optimized for a single application because each application has different requirements for at least a) data rate, b) bit error rate, c) latency, d) symmetry/asymmetry of transmission and e) immunity to impulse noise and other transient phenomena. As a result, a transceiver that is optimized for one application, such as video-on-demand, does not work as well if used for a different application, such as Internet access.
Classic transceiver architecture includes a Framer/Coder/Interleaver (FCI) block, a digital modulation block, an Analog Front End (AFE) block, and a communications channel. In most cases, it is the FCI block that is optimized for a specific application because the FCI block has significant control over three of the four parameters mentioned above: bit error rate, latency and immunity to impulse noise. Many transceivers use forward error correcting (FEC) codes, block and/or convolutional codes, to improve the bit error rate (BER) performance. Combining large block FEC codes with interleavers provides immunity to impulse noise. The disadvantage of large block FEC codes and interleaving is that they add latency to the system. As an example, if low BER and immunity to impulse noise are required for a specified application, the transceiver may include FEC codes and interleaving. If for another application low latency is important but a higher BER and/or burst errors resulting from impulse noise are tolerable, convolutional codes and no interleaving may be used.
The Digital Modulation block, the AFE block and the transmission channel establish the data rate of the transceiver for a specified BER and margin. Advancements in signal processing techniques and silicon processes in the digital modulation block and AFE technology have led to significant improvements in the data rates achievable on twisted pair phone lines. The result of this dramatic increase in transmission bandwidth is the ability to transport multiple applications over a single transceiver connection. Given the different performance requirements of different applications, there is a need for a transceiver designed to transmit and receive for multiple applications. Furthermore, there is a need for a transceiver designed to dynamically switch from transmitting and receiving data for a first set of applications to transmitting and receiving data for a second different set of applications.